The effects of long term hemodynamic overload in middle-aged or older animals is unclear because most studies have used relatively young animals or, when older ones were used, did not compare different age groups with the same duration of the hemodynamic load. There is good reason to believe that the adaptive process of hypertrophic growth may be modified by the age of the animal. Atrioventricular block (B) causes a volume overload state that can be tolerated for long periods of time and induces considerable cardiac hypertrophy that can be related to the amount which the heart rate is slowed. To examine the effect of this type of chronic hemodynamic overload in animals of different ages, we used closed chest electrocautery to induce complete B in male, Sprague- Dawley rats of 5,12, and 16 months of age for mean duration of 7 months. The results indicate aging over this adult period modified the influence of long term B on cardiac size and the function of isolated cardiac muscle. Chronic B: (1) caused cardiac hypertrophy that was greater in the younger than older animals; (2) prolonged contraction duration that was correlated with the extent of hypertrophy regardless of age and (3) resulted in age-related alterations in contractile function of isolated intact and "chemically skinned" muscle, with the youngest B have more force than control (C) and the older B having equal or less force than C. Differences between younger and older B in both excitation- contraction coupling and myofilament force production are required to explain the observed results. These findings indicate an age- related difference in the chronic myocardial adaptive response to chronic atrioventricular B. The age-related differences in force development in the present study cannot be attributed to the age- related differences in hypertrophy. The ultimate explanation for these findings will require a better understanding of how the myocardium "restructures" itself under the influence of any given hemodynamic stress and how aging affects this process.